Bollinger Bands in MetaTrader 5 — MQL5 Source Code

MetaTrader ships Bollinger Bands, but building your own is the cleanest way to learn how MT5 handles multi-buffer overlays — and to be certain your bands use the correct (population) standard deviation. See the explainer for what we’re drawing.

What you’ll need

• MetaTrader 5 with MetaEditor
• Save as AlgoGen.Bollinger.mq5 under MQL5/Indicators/

The source

Three plotted buffers — middle, upper, lower — computed with a simple average and the population standard deviation over the same window.

//+------------------------------------------------------------------+
//|                                            AlgoGen.Bollinger.mq5 |
//|                                          Copyright 2026, AlgoGen |
//|                                           https://www.algogen.io |
//+------------------------------------------------------------------+
#property copyright "Copyright 2026, AlgoGen"
#property link      "https://www.algogen.io"
#property version   "1.00"
#property indicator_chart_window
#property indicator_buffers 3
#property indicator_plots   3

#property indicator_label1  "Middle"
#property indicator_type1   DRAW_LINE
#property indicator_color1  clrOrangeRed
#property indicator_width1  2
#property indicator_label2  "Upper"
#property indicator_type2   DRAW_LINE
#property indicator_color2  clrMediumPurple
#property indicator_label3  "Lower"
#property indicator_type3   DRAW_LINE
#property indicator_color3  clrMediumPurple

input int    InpPeriod = 20;   // Period
input double InpMult   = 2.0;  // Std-dev multiplier

double MidBuffer[], UpBuffer[], LoBuffer[];

int OnInit()
  {
   SetIndexBuffer(0, MidBuffer, INDICATOR_DATA);
   SetIndexBuffer(1, UpBuffer,  INDICATOR_DATA);
   SetIndexBuffer(2, LoBuffer,  INDICATOR_DATA);
   for(int p = 0; p < 3; p++)
      PlotIndexSetInteger(p, PLOT_DRAW_BEGIN, InpPeriod - 1);
   IndicatorSetString(INDICATOR_SHORTNAME,
      StringFormat("AlgoGen BB(%d,%.1f)", InpPeriod, InpMult));
   return(INIT_SUCCEEDED);
  }

int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],  const double &open[],
                const double &high[],    const double &low[],
                const double &close[],   const long &tick_volume[],
                const long &volume[],    const int &spread[])
  {
   if(rates_total < InpPeriod)
      return(0);

   int start = (prev_calculated > InpPeriod) ? prev_calculated - 1 : InpPeriod - 1;

   for(int i = start; i < rates_total; i++)
     {
      double sum = 0.0;
      for(int j = 0; j < InpPeriod; j++)
         sum += close[i - j];
      double mean = sum / InpPeriod;

      double sqsum = 0.0;
      for(int j = 0; j < InpPeriod; j++)
        {
         double d = close[i - j] - mean;
         sqsum += d * d;
        }
      double sd = MathSqrt(sqsum / InpPeriod);  // population std (divide by N)

      MidBuffer[i] = mean;
      UpBuffer[i]  = mean + InpMult * sd;
      LoBuffer[i]  = mean - InpMult * sd;
     }
   return(rates_total);
  }
//+------------------------------------------------------------------+

Install & compile

1. Drop it in MQL5/Indicators/, press F7 to compile.
2. Drag onto a chart — three lines overlay price, matching the output chart.

Gotchas

• Divide by N, not N−1. sqsum / InpPeriod gives the population standard deviation Bollinger Bands require. Dividing by InpPeriod - 1 (sample std) makes the bands too wide — the same trap as pandas’ ddof.
• The built-in shortcut. In an EA you can use iBands(...) and CopyBuffer. MT5’s built-in already uses population std, so this hand-rolled version matches it — good confirmation your loop is right.
• MQL4? Same math; swap the OnCalculate signature and buffer setup, or call iBands.

Same indicator elsewhere: Python, Pine Script, EasyLanguage, NinjaScript. Then backtest it in AlgoGen.

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This post is educational, not financial advice. Indicators describe the past; they don’t predict the future. Backtest anything before you risk real money on it.